1. Field of the Invention
The present disclosure generally relates to image forming and, more particularly, to an image forming apparatus having a function of measuring color of a measuring image.
2. Description of the Related Art
An image forming apparatus has image qualities such as granularity, in-plane uniformity, character quality, and color reproducibility (including color stability). With the proliferation of multicolor image forming apparatuses today, color reproducibility is sometimes said to be the most important image quality.
People have their empirically-grounded memories of expected colors (like human skin, blue sky, and metals), and exceeding an allowable range would give a sense of strangeness. Such colors are referred to as memory colors, whose reproducibility is being required more often when outputting photographs.
Office users are experiencing a sense of strangeness with document images as well as photographic images, in the presence of a color difference from a display monitor. Graphic arts users are pursuing color reproducibility of computer graphics (CG) images. Color reproducibility (including stability) demanded of image forming apparatuses by the above users is ever increasing.
To meet the users' demand for color reproducibility, for example, Japanese Patent Application Laid-Open No. 2004-086013 discusses an image forming apparatus that reads measurement images (patch images) formed on a sheet with a measurement unit (color sensor) arranged on a conveyance path of the sheet. According to the image forming apparatus, a process condition including the amount of exposure and a development bias can be feedback-controlled based on the reading result of the patch images by the color sensor, whereby a constant density, gradation, and tint can be reproduced.
The color detection accuracy of the color sensor discussed in Japanese Patent Application Laid-Open No. 2004-086013 deteriorates due to factors such as variations of the output of a light source due to a change in the ambient temperature. A white reference plate then may be arranged in a position opposed to the color sensor, so that the color sensor can measure the white reference plate and correct detected values of the color sensor.
Specifically, a spectral reflectance R(λ) of a patch image can be determined by the following equation:R(λ)=P(λ)/W(λ),  (Eq. 1)where W(λ) is the reflected light amount from the white reference plate, and P(λ) is the reflected light amount from the patch image.
The determination of the spectral reflectance R(λ) by using the white reference plate has a problem in that the measured spectral reflectance R(λ) can contain errors depending on a change in the state of the white reference plate such as a deterioration and stain of the white reference plate. For example, if the white reference plate undergoes a change because of tint variations of the white reference plate due to aged deterioration or the adhesion of a stain to the white reference plate, the original reflectance W(λ) of the white reference plate is erroneously detected as W′(λ). As a result, the spectral reflectance R(λ) of the patch image is erroneously calculated as R′(λ).